Dot-sequential color display system

ABSTRACT

1266-03028 A color display system includes a color light separator that separates incident white illumination light into red, green and blue wavelength bands to be directed to distinct color component sub-pixels (sometimes called dots) that are arranged in a dot-matrix, color triad arrangement (e.g., stripe or delta) to form individual picture elements (pixels) on a pixelated electronic image device (e.g., LCD of DMD). The entire picture is optically shifted from one set of color component sub-pixels to another in a 3-field sequence. As a result, the sets of red, green and blue color component sub-pixels appear to an observer as a single full-color image, thereby providing a dot sequential color display.

SUMMARY OF INVENTION

[0001] There are many ways to produce a full color matrix-addresseddisplay, but almost all methods require 3 independent elements of Red,Green and Blue coloration, so as to be able to mix (in the additivecolor method) each primary in variable ratios to be able to cover theentire color gamut.

[0002] One method that does not need 3 simultaneous primary colorelements is referred to as field-sequential color. In this method animaging device is illuminated with just one color primary at a time. Onecan envision a color-wheel that filters the white light and allowspassage of first red, then green, then blue light, to be directed ontothe imager, and if the sequencing is fast enough the human eye willintegrate these R, G, B subimages into one full color image. In theearly days of television it was proposed to use field-color sequentialdisplay, with a rotating color wheel in front of a white (broadband)emissive display (e.g. CRT) and cycle between red, green, and blue colorsubimages, and let the human eye-brain synthesize these 3 primary colorimages to one full-color image.)Very recently the use offield-sequential color has been revived, with the Texas InstrumentsDigital Micromirror Device (DMD), wherein a single proprietary(expensive) DMD chip can be used with a quickly rotating color wheel tocreate 180 color fields per second and therefore 60 full-color framesper second. In order to alleviate the flickering effect of thissequential color display, the color wheel might rotate still faster, toprovide 120 full-color frames per second.

[0003] The major drawback of this type of display is that one-third ofthe light emanating from the display (in the case of a single-chip DMD,transmitted through the color wheel and reflected off of the micromirrorelements) is used at any one time, and due to a “dead-band” requiredbetween the color segments (to prevent any color cross-talk) this may befurther reduced to around 30% light utilization efficiency.

[0004] Whereas all displays appear to be improved with increasedbrightness, this potential 30% efficiency is a serious detriment.

[0005] Another recent attempt at producing a color-sequential deviceconcentrates three bands of light (R, G, and B stripes) onto a singledisplay and “scrolls” these colors dynamically to produce a higherefficiency. The drawback of this “scrolling color” method is thebulkiness of a scanner to scroll the illumination and awkwardness ofaddressing the imager in a somewhat arbitrary (random access) manner asopposed to simple progressive line manner.

[0006] Shimizu of N. Am. Philips Labs has presented papers at displayconferences on this idea that promises to use nearly 100% of the light,but requires that three sub-bands of R, G and B light be incident on thesingle imaging device and (barber-pole-wise) scrolled so that just asthe B (e.g.) light segment finishes lighting the bottom of the display,it is starting to light up the top of the display and G and R (e.g.) areright behind in continuous sequence. This is a very difficult opticaltask, to “scroll” the three separate light source color bands onto theLCD.

[0007] This invention utilizes all of the light all of the time, i.e.the light utilization efficiency may be nearly 100% (disregardingtypical light collection losses which all systems have to some extent).

[0008] In the present invention includes a color separation means sothat R, G and B wavelength bands are directed to distinct colorcomponent sub-pixels (sometimes called dots) that are arranged in adot-matrix, color triad arrangement (e.g., stripe or delta) to formindividual picture elements (pixels). The entire picture is opticallyshifted from one set of color component sub-pixels to another in a3-field sequence. As a result, the sets of R, G, and B color componentsub-pixels appear to an observer as a single full-color image. Thisinvention is sometimes called a dot sequential color display.

[0009] This small shifting of the picture must take place rapidly (i.e.at a fast field rate) but will be much less noticeable than the completechange of color that accompanies the typical field sequential colordisplay. The dot sequential display of this invention differs from afield sequential display in that the former uses different sub-pixels ordots for each color component, whereas the latter successively uses thesame pixels for each color component. The field sequential displaysuffers from a macro-color flicker effect that is very noticeable unlessthe field rate is much higher than 180 Hz.

[0010] In accordance with a preferred embodiment of the invention, weuse a pixellated display (typically a liquid crystal display device,LCD) at a particular resolution, for example 900×600 dots and from this“monochrome” device we create a full color display.

[0011] First we create a color display, with one-third the resolution,e.g. 300 (×3)×600 full-color dots and then “sequentially”, on a field byfield basis, we displace these 900 dots left and right (for instance) sothat R, G and B overlap in space to create a 900 full-color dot image.In conventional display applications, such as electronic (e.g., LCD)display projectors, the pixellated display will have a resolution thatcorresponds to the desired final overall display resolution (e.g.,640×480, 800×600, 1024×768, 2048×1536, etc,) Dot-sequential Color uses asingle LCD (or DMD, etc.) imaging device at the desired finalresolution, but creates 3 slightly displaced images over time, to makethe equivalent of a full-color image with very low cost. We thereforeexploit the ability of the human eye to synthesize three displaced colorimages into an equivalent higher resolution color image. Such adisplacement may also be called “dithering” or “dot dither”.

[0012] Other prior art projection display systems use 3 LCDs in anoptical system that separates the projection illumination into R, G andB paths, and then after illuminating the 3 independent full-resolutiondevices, these R, G and B images are superimposed at the viewing screenor observer's eye.

[0013] Another popular display system uses color filters within thedisplay but does not separate the illumination into the separate pixelsbut rather crudely forces the white light through the red filter, thuslosing {fraction (2/3)}rds of the incident light, with a similar 30%light efficiency as field-sequential color displays.

[0014] U.S. Pat. No. 5,969,832 (Nakanishi et al) proposes a display withhigh efficiency usage of light, but which moves the illumination intothe RGB subpixels instead of keeping the colors constant as ourinvention requires. One problem that may be encountered when using themethod of the '832 patent with a liquid crystal can be described brieflyas follows. Imagine a red ball moving across a black background: Whenthe red ball moves and a red pixel is turned on to pass light, and nowwe switch the illumination into this same pixel so that it must passgreen light, until such time as the red liquid crystal element takes to“turn off” the light, there will be some green light passing through,which will have produce an artifact of some slight amount of green lightin locations throughout the area of the red ball, where there should beNO green light. Likewise there will be blue edges also appearing on thered ball which will be increasingly obvious as it moves around.

[0015] Furthermore the system of the '832 patent does not have arealistic and inexpensive means of shifting the illumination on a fieldrate (i.e. 60 Hz or faster) basis. The '832 patent requires that R, Gand B wavebands be separated and directed into RG and B subpixels, and adisplay of {fraction (1/3)} resolution is recommended, but theillumination is shifted into these pixel groups, instead of ourinvention which shifts the image after the illumination is steadilyapplied. Note that in our invention the red light is steadily applied toa “red” pixel and the error that accrues due to non-instantaneous LCresponse time causes incorrect amplitudes of red light instead ofcross-color contamination (e.g. some green or blue light, when the imagecalls for only red and black). In our method there could be an imperfectred ball edge amplitude (in fact the edges of the ball may be softenedas is desirable for reducing jaggedness of the pixellated image).

[0016] The preferred method of shifting the image of our device uses asimple tilted plate with a piezoelectric actuator. Another means ofobtaining such a small image displacement uses a double-birefringencecrystal and liquid crystal retarder which is switched between twopolarization states to make a “solid state” and reliable andfast-switching displacement control device.

[0017] In U.S. Pat. No. 5,537,256 (for example) Fergason describes ameans of dynamically displacing an image, by means of an LC switch anddoubly-birefringent device. U.S. Pat. No. 5,161,042 (Hamada) discloses amethod which increases the light efficiency similar to the here-indescribed invention, but fails to show how to use a lower resolution LCDwith sequential displacement to increase the effective resolution.

[0018] In U.S. Pat. No. 5,467,206 there is disclosed a holographic colorseparation system which shows the same intention as Hamada, i.e. tochannel the RGB wavelength bands into three sets of pixels, but usingholographic dispersion and a holographically formed microlens. Whilesuch a system is relatively easy to implement, it suffers the sameproblem as Hamada's tilted dichroic and microlens array approach, i.e.the light for the outside two channels becomes widely diverging and theprojection lens must be large and the overall image quality suffers as adirect result.

[0019] Our chromatic separation also preferably uses a holographicgrating, but as a compensating dispersion, and not as a microlens. Wepropose a better solution to putting RG and B wavelengths into spatiallydistinct pixel regions (RGB subpixels) and we furthermore propose a moreeconomical method of displacing the image in time. Our display systemand method provides high-efficiency illumination and a high-resolutionimage to the human eye.

[0020] Additional objects and advantages of the present invention willbe apparent from the detailed description of the preferred embodimentthereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is an optical schematic illustration of a dot sequentialcolor display system according to the present invention.

[0022]FIG. 2 is a diagrammatic illustration of color component pixeladdressing in a conventional prior art field sequential display system.

[0023]FIG. 3 is a diagrammatic illustration of color component pixeladdressing in a dot sequential display system according to the presentinvention.

[0024]FIG. 4 is a diagrammatic time-sequential illustration of oneimplementation by which the portions of a color component plane aredelivered to a display device at successive times.

[0025]FIG. 5 is a diagrammatic illustration of cumulative successivetimes during which portions of all three color component planes aredelivered to a display device during one image frame.

[0026]FIGS. 6 and 7 are optical schematic illustrations of alternativeimplementations of a dot sequential color display system with an angularcolor separation system.

[0027]FIG. 8 is an enlarged side view illustrating light propagatingthrough a prism array.

[0028]FIG. 9 illustrates renormalization of RGB light channels asfacilitated by the prism array of FIG. 8.

[0029]FIG. 10 is an optical schematic illustration of a diffractivecolor filter that includes a holographic grating.

[0030]FIG. 11 is an optical schematic illustration of another dotsequential color display system according to the present invention.

[0031]FIG. 12 is an alternative optical schematic illustration of adiffractive color filter with a holographic optical element grating.

[0032]FIG. 13 is an exploded functional illustration of a holographicoptical element grating functioning as a uniform telecentric colorseparator.

[0033]FIG. 14 is an optical schematic illustration of a resolutionenhancing dot sequential color display system according to the presentinvention.

[0034]FIG. 15 illustrates resolution enhancement provided by the displaysystem of FIG. 14.

[0035]FIG. 17 further illustrates resolution enhancement provided by thedisplay system of FIG. 14.

[0036]FIG. 18 is a diagrammatic illustration of a dynamic post-displaypixel element alignment system or “wobbler” that includes two prismarrays with piezoelectric actuator stacks therebetween.

[0037]FIG. 19 illustrates a dynamic post-display pixel element alignmentsystem or “wobbler” that includes a flat doubly birefringent crystal.

[0038]FIG. 20 is a front view of a dynamic post-display pixel elementalignment system or “wobbler” that includes a wheel with four flatrefractive segments.

[0039]FIG. 21 is a side view of the wheel of FIG. 20.

[0040]FIG. 22 is a top view of the wheel of FIG. 20.

[0041]FIG. 23 is a diagrammatic illustration of an alternative colormosaic arrangement of sub-pixels in a display device (e.g., LCD).

[0042]FIG. 24 is a diagrammatic illustration of another alternativecolor mosaic arrangement of sub-pixels in a display device (e.g., LCD).

DETAILED DESCRIPTION

[0043] The present invention relates to pixelated electronic (e.g.,liquid crystal display, digital micromirror device, etc.) projectiondisplays, sometimes referred to as electronic display projectors. Theinvention includes a dot sequential color display system that may beused in such an electronic display projector. It will be appreciated,however, that the dot sequential color display system of the presentinvention could alternatively be used in other display applications.

[0044]FIG. 1 is an optical schematic illustration of a dot sequentialcolor display system 10 according to the present invention. A parabolicreflector 12 collects generally white light from a lamp 14 (e.g., an arclamp) and directs the light in generally parallel rays to a firstgrating 16. Grating 16 disperses or separates color components of thelight (e.g., red, green and blue “RGB”) and directs them to a microlensarray 18 that focuses the dispersed light onto or toward a pixelatedelectronic display (e.g., a liquid crystal display) 20. A second grating22 re-normalizes the angle of incidence for the color components (e.g.,RGB), thereby compensating for the dispersion imparted by grating 16.

[0045] LCD 20 includes triads of color component sub-pixels forcontrolling the intensity of each color component of light (e.g., RGB).A wobbler or “dot-shifter,” such as a dynamically tilted plate 24, istilted at a fast field rate to form a three-frame sequence. Plate 24 isdynamically tilted, “wobbled,” or dithered in synchronism with theapplication of color component image signals to LCD 20 to direct threeoverlapping images of color component sub-pixels or dots to a projectionlens assembly 26. Projection lens assembly 26 projects the overlappingimages of color component sub-pixels or dots onto a display screen 28that is viewed by one or more observers.

[0046]FIG. 2 is a diagrammatic illustration of color component pixeladdressing in a conventional prior art field sequential display system30. Display system 30 includes a frame buffer memory 32 with red, green,and blue color component planes 34 that store at each of multipleaddresses or locations 36 a value corresponding to the intensity of acolor component for a pixel of a display image. Although color componentplanes 34 are illustrated as being separated for each of the red, green,and blue color components, it will be appreciated that in manyimplementations the locations 36 of the color component planes 34 areinterleaved in the physical memory structure where the color componentvalues are stored.

[0047] At successive times t1 , t2, and t3, the color componentinformation for a corresponding single color component plane 34 isdelivered to and a corresponding image is rendered by a pixelateddisplay device 38, such as a liquid crystal display or a digitalmicromirror device. The pixels 40 of display device 38 have a one-to-onecorrespondence with locations 36 in frame buffer memory 32. For example,frame buffer memory 32 with color component planes 34 having j-by-k(columns-by-rows) arrays of addresses 36 will correspond to displaydevice 38 having an x-by-y array of pixels 40. In one common displayformat, the j-by-k arrays of addresses 36 and the x-by-y array of pixels40 may correspond to 1024-by-768. It will be appreciated that the arraysof addresses in some frame buffer memories may be of a size differentthan (e.g., typically larger than) the array of pixels in the displaydevice. In these situations, the above description is directed to thematching portions of the frame buffer memory and display device arrays.

[0048] For purposes of illustration, display device 38 is illustrated asreceiving and rendering the red, green, and blue color components atsuccessive times t1, t2, and t3, respectively. At each time t1, t2, ort3, display device 38 functions to render monochrome red, green, or blueimage information. The red, green, and blue color components rendered atsuccessive times t1, t2, and t3 by display device 38 are superimposed ona display screen to form an image.

[0049]FIG. 3 is a diagrammatic illustration of color component pixeladdressing in a dot sequential display system 50 according to thepresent invention. Display system 50 includes a frame buffer memory 52with red, green, and blue color component planes 54 that store at eachof multiple addresses or locations 56 a value corresponding to theintensity of a color component for a pixel of a display image. Althoughcolor component planes 54 are illustrated as being separated for each ofthe red, green, and blue color components, it will be appreciated thatin many implementations the locations 56 of the color component planes54 are interleaved in the physical memory structure where the colorcomponent values are stored.

[0050] At successive times t1, t2, and t3, a portion (e.g., one-third)of the color component information for each of the color componentplanes 54 is delivered to and a corresponding partial image is renderedby a pixelated display device 58, such as a liquid crystal display or adigital micromirror device. The pixels 60 of display device 58 have aone-to-one correspondence with a portion (e.g., one-third) of locations56 in frame buffer memory 52. For example, frame buffer memory 52 withcolor component planes 54 having j-by-k (columns-by-rows) arrays ofaddresses 56 will correspond to display device 58 having an x-by-y arrayof pixels 60. In one common display format, the j-by-k arrays ofaddresses 36 may correspond to 1024-by-768, and the x-by-y array ofpixels 40 may correspond to 1024-by-768. Color component information inan array of about 1024/3-by-768 addresses 56 from each color componentplane 54 is delivered to display device 58 at each time t. It will beappreciated that the arrays of addresses in some frame buffer memoriesmay be of a size different than (e.g., typically larger than) the arrayof pixels in the display device. In these situations, the abovedescription is directed to the matching portions of the frame buffermemory and display device arrays.

[0051] At each time t1, t2, or t3, display device 58 functions to rendera portion (e.g., one-third) of the full-color image information. Thesepartial full-color images are distinct from the successive monochromeimages formed in conventional field sequential system 30. The partialfull-color images rendered at successive times t1, t2, and t3 by displaydevice 58 overlap and are interleaved on a display screen to form animage 62.

[0052]FIG. 4 is a diagrammatic time-sequential illustration of oneimplementation by which the portions of a color component plane 54(e.g., green) are delivered to display device 58 at successive times t1,t2, and t3. For purposes of simplicity, this illustration shows only asmall fraction (e.g., 6 columns) of the typically many more of columnslocations 56 and pixels 60 in color component plane 54 and displaydevice 58, respectively. Moreover, this description of green colorcomponent plane is similarly applicable to the red and blue colorcomponent planes 54.

[0053] At a step 70 corresponding to a time t1, color componentinformation in every third column of addresses or locations 56 in greencolor component plane 54 is delivered to every corresponding thirdcolumn of pixels 60 in display device 58. In the illustration of step70, for example, columns j and j+3 of locations 56 in green colorcomponent plane 54 are delivered to corresponding columns x+1 and x+4 ofpixels 60 in display device 58.

[0054] At a step 72 corresponding to a time t2, color componentinformation in every next successive third column of locations 56 ingreen color component plane 54 is delivered to every corresponding thirdcolumn of pixels 60 in display device 58. In the illustration of step72, for example, columns j+1 and j+4 of locations 56 in green colorcomponent plane 54 are delivered to corresponding columns x+1 and x+4 ofpixels 60 in display device 58.

[0055] At a step 74 corresponding to a time t3, color componentinformation in every next successive third column of locations 56 ingreen color component plane 54 is delivered to every corresponding thirdcolumn of pixels 60 in display device 58. In the illustration of step72, for example, columns j+2 and j+5 of locations 56 in green colorcomponent plane 54 are delivered to corresponding columns x+1 and x+4 ofpixels 60 in display device 58.

[0056] The operations described for green color component plane 54 aresimultaneously carried out for red and blue color component planes 54.The color component information in every third column of addresses orlocations 56 in green color component plane 54 is successively deliveredto every corresponding third column of pixels 60 (e.g, columns x+1 andx+4) in display device 58. Similarly, the color component information inevery third column of addresses or locations 56 in red color componentplane 54 is successively delivered to every corresponding third columnof pixels 60 (e.g, columns x and x+3) in display device 58, and thecolor component information in every third column of addresses orlocations 56 in blue color component plane 54 is successively deliveredto every corresponding third column of pixels 60 (e.g, columns x+2 andx+5) in display device 58. Accordingly, each column of pixelsconsistently receives color component information of only one colorcomponent.

[0057]FIG. 5 is a diagrammatic illustration of the cumulative times t1,t2, and t3 during which portions of all three color component planes 54are delivered to display device 58 during one image frame. Displaydevice 58 illustrates that at each of times t, pixel columns x and x+3receive blue color component information, pixel columns x+1 and x+4 ofdisplay device 58 receive green color component information, and pixelcolumns x+2 and x+5 of display device 58 receive red color componentinformation.

[0058] Wobbler 24 illustrates its positions at times t1, t2, and t3 andalso the manner in which wobbler 24 superimposes on a display screen 76red, green, and blue color components rendered at successive times t1,t2, and t3 to form three exemplary pixels 78 (m, m+1, and m+2) of adisplay image. TABLE 1 [t1] Time Display Device Pixels Display ScreenPixels t1 x + 1 x + 2 x + 3 m m + 1 m + 2 Green Red Blue Green Red Bluet2 x x + 1 x + 2 m m + 1 m + 2 Blue Green Red Blue Green Red t3 x − 1 xx + 1 m m + 1 m + 2 Red Blue Green Red Blue Green

[0059] Dot sequential color display system 10 includes high efficiency,color-separated, fixed illumination of color-component sub-pixels in LCD20 and dynamic post-display device alignment of the color-componentsub-pixels. The color-separated, fixed illumination of color-componentsub-pixels in LCD 20 includes splitting the generally white projectionlight into 3 (or 4) primary color wavelength bands and directing eachseparate band to separate sub-pixel elements arranged in a color mosaicor color-stripe pattern on LCD 20. Various means can be used to separatethe generally white light into color components, including: 1.absorptive (lossy) color filter triads (e.g., Fergason U.S. Pat. No.5,715,029)—very poor efficiency 2. angular color separation (ACS) (e.g.,Hamada, U.S. Pat. No. 5,161,042) 3. holographic Acs (e.g., Huignard etal. U.S. Pat. No. 5,467,206) 4. telecentric approach (e.g., NishiharaU.S. Pat. No. 5,764,319) 5. telecentric filter, microlens+HOE(holographic optical element), described hereinbelow.

[0060] The dynamic post-display device re-alignment of thecolor-component sub-pixels (i.e., wobbling or dithering) includesdisplacing the image to the eye by a subpixel element in time, over 3(or 4) field time periods so as to superimpose the color dots on top ofeach other to realize full color dots. Various wobbling or ditheringmeans can be used, including: 1. Liquid crystal switch and birefringentcrystal (e.g., Fergason U.S. Pat. No. 5,715,029) 2. piezoelectricactuators between symmetrical prism arrays, as described below 3.solenoid or piezos to tilt a plate, as described below 4. othermechanical means (e.g., CCD dithering).

[0061] The present system and method include separating the colorcomponents into mosaic color primary picture elements (on theillumination side of the display) and then subsequently superimposingthese elements to the eye (on the viewing side of the display). It maybe seen that each of these two processes can be accomplished by avariety of means.

[0062] Prior displays described by Fergason (U.S. Pat. No. 5,715,029)and by Nakanishi (U.S. Pat. No. 5,969,832) do not realize the presentinvention. Fergason suffers significant inefficiency; throwing away{fraction (2/3)} of the illumination light by using absorptive colorfilters. Nakanishi requires that the illumination be shifted, which isparticularly difficult because the illuminator typically has asignificant mass that can be difficult to shift or displace at anadequate frequency, particularly in comparison to the significantlylower mass of a wobbler 24 of the present invention. Moreover, shiftingor displacement of the illuminator would make it particularly difficult,if not impossible, to precisely fill or illuminate the pixel aperturesof the separate color component light channels. Finally, cross-colorcontamination could be introduced by such illumination shifting due todisplay devices (e.g., LCDs) having less than idealized response times.The present invention is much more practical, shifting the display imagebut statically placing color illumination light into separate dedicatedcolor subpixel elements.

[0063]FIGS. 6 and 7 are optical schematic illustrations of alternativeimplementations of this invention in which grating 16 of dot sequentialcolor display system 10 is replaced with an angular color separationsystem 90 of the type described in U.S. Pat. No. 5,161,042 of Hamada.The implementation of FIG. 7 further includes a prism array 92 thatfunctions as a total internal reflection (TIR) ‘deflector’ that receivesnormal incident light and deflects or angles the light to some desireddirection so as to be appropriate for the next stage. FIG. 8 is anenlarged side view illustrating light propagating through prism array92.

[0064] In the implementations of FIGS. 6 and 7, grating 22, whetherholographic or not, is positioned between microlens array 18 and thepixel apertures of LCD 20. This allows the incident RGB light channelsto be renormalized (as illustrated in FIG. 9) and go through the RGBpixel apertures at approximately the same (0 degree) angle. For example,a holographic grating 22 may be positioned midway between microlensarray 18 and the pixel elements of LCD 20. The renormalization that thisprovides is the same as that provided by two layers of microlenses or amicrolens and a microprism (see for example U.S. Pat. No. 5,764,319 ofNishihara), but with a larger angular acceptance angle and an easier,more exacting renormalization.

[0065] In contrast, angular color separation systems of the typedescribed in U.S. Pat. No. 5,161,042 of Hamada have output angles thatare widely diverging with a center channel on-axis but two outerchannels emanating to extreme directions left and right from that. Insuch prior systems, the dispersive (e.g. holographic) element ispositioned first, then a microlens near the LCD. U.S. Pat. No. 5,467,206also contemplates making the hologram perform as a microlens. Thepresent invention places the holographic diffractive element between themicrolens and the pixel apertures, in contradistinction to the '206patent in which the microlens is placed between the HOE and the pixel.

[0066]FIG. 10 is an optical schematic illustration anotherimplementation in which a color-dispersing element (e.g., a grating orACS of Hamada, not shown) separates the light into 3 different angularchannels for three distinct wavelength ranges—red, green and blue. Thesethree channels pass in sequence through a color filter 93 that includesa refractive (i.e., not holographic) lens array 94 (which may preferablybe an array of cylindrical lenses, i.e. a lenticular) and a holographicgrating 96 to an imaging device 98 (e.g., LCD), such that the averageangle for all three exiting channels (i.e. R, G and B) is made to besubstantially normal to the imaging plane of imaging device 98. Asillustrated in FIG. 10, this provides a telecentric configuration inwhich the pixel apertures of imaging device 98 are located at the frontfocus, resulting in the chief rays being parallel to the optical axis inthe image space (i.e., normal to the plane of imaging device 98).

[0067] This arrangement of dispersive (color-separating) element plus(refractive) lens array element 94 plus holographic (counter-dispersive)element 96 is unique and has the important feature that final grating 96is continuous and without any optical power. As a result, final(holographic) grating 96 need not be aligned to the pixels of imagingdevice 98 other than to ensure that the grating axis is parallel withthe columns of the display pixels. The micro-lenticular 94 is carefullyaligned, but this may be added onto the built and tested imaging device98 as a secondary process step, which would increase the manufacturingyield rate for both imaging device 98 and lenticular 94. The“dispersion-compensating” element 96 is preferably a volume hologram soas to be able to be immersed and which has a high diffraction efficiencyover the narrow angles that it is designed to accept. In contrast, asurface grating cannot be glued between glass or plastic optical layers.

[0068] In one embodiment, the first color dispersive element isidentical to the final counter-dispersive element 96 and both are volumeholographic transmission gratings. From the standard simplified gratingequation:lambda divided by “d”=sin(input angle)+sin(output angle), inwhich lambda is the wavelength of light and d is the grating spacing.The ideal input angle can be selected based on an arbitrary colorchannel separation angle. In the case of the output angle being normalto the holographic plane, and for lambda for green light ofapproximately 0.55 microns wavelength, the formula may also reveal anominal grating spacing, “d”:“d”=0.55 divided by sin(input angle)orlikewise input angle=arcsin (d/lambda) Table 2 below shows variouschannel spacings as a function of incident (input) green light.) TABLE 2[t2] Channel Red incident Green Blue Grating separation angle incidentincident spacing (delta R-G) (degrees) angle angle (microns) 8 49 41 330.83 6 39 33 27 1.0  4 28 24 20 1.35 3 21 18 15 1.75 2   14.7   12.7  10.7 2.5 

[0069] Note that spatial frequency is inverse of “d”. Multiply by 1000to get cycles/mm. Therefore a grating spacing of 1.75 micronscorresponds to 571 cycles/mm.

[0070] In a specific application, a suitable LCD is selected, with aspecific pixel spacing “p” and glass substrate thickness “h”: Theholographic element is desirably a volume grating, as opposed to surface(replicated, etc) structure. This volume grating suppresses the higherdiffraction orders so that principally one channel for each R, G and Bbundle is obtained, not multiple random paths. The off-axis higherorders can probably be eliminated within the projection lens, since itwould tend to vignette such extreme angles.

[0071] The central angle (i.e. for 550 nm green light) may be adjustedas required to match the LCD pixel pitch (spacing between R, G, Bsubpixels) and the thickness of LCD substrate glass and the HOEthickness. The grating functions to straighten out each of thewavelength bands, so the input angles are designed arbitrarily to getmaximum efficiency for red (e.g., 632 nm), green (e.g., 546 nm) and blue(e.g., 480 nm). A specific LCD of interest has 0.7 mm glass and asubpixel pitch of 42 microns. A simple ray trace gives a nominal designwith 2 degrees between each color primary, so the HOE can be made to doa ‘normalization’ for green light. The central illumination angle forgreen is then designed so that red and are blue diffracted on eitherside at about 2 degrees. In this case the nominal input angle is 15degrees, and the spatial frequency of the transmission (holographic)grating is about 1000 cycles per mm.

[0072] In one implementation, there may be some refinement since thediffraction is not linear. The photopic response of the eye makes itless important to worry about blue light below 450 nm and red lightabove 660 nm, and the lamp has its own unique spectral signature.Generally the yellowish peak (Hg has strong 579 line) is attenuatedsince this light contaminates both red and green color primary channels.

[0073] The microlens may be refractive and needs to be precisely alignedwith the LCD. The holographic grating should be relatively thick tosuppress higher orders and yet structurally thin so that the spacingfrom microlens to LCD pixel can be minimized to increase the angularacceptance for the illumination. The hologram not need have any lensfunction, counter to the teaching of U.S. Pat. No. 5,467,206.

[0074]FIG. 11 is an optical schematic illustration of another dotsequential color display system 100 according to the present invention.A parabolic reflector 112 collects white light from a lamp 114 (e.g., anarc lamp) and directs it in parallel rays to an angular color separation(ACS) system 116. ACS system 116 disperses or separates color componentsof the light and directs them to a microlens array 118 that focuses thedispersed light onto a pixellated electronic display (e.g., a liquidcrystal display) 120. A holographic optical element 122 re-normalizesthe angle of incidence for the color components (e.g., RGB), therebycompensating for the dispersion imparted by ACS system 116.

[0075] LCD 120 includes triads of color component sub-pixels forcontrolling the intensity of each color component of light (e.g., RGB).A “dot-shifter,” such as a dynamically tilted plate 124, is tilted at afast field rate to form a three-frame sequence. Plate 124 is dynamicallytilted, “wobbled,” or dithered in synchronism with the application ofcolor component image signals to LCD 120 to direct three overlappingimages of color component sub-pixels or dots to a projection lensassembly 126. Projection lens assembly 126 projects the overlappingimages of color component sub-pixels or dots 128 onto a display screen(not shown) that is viewed by one or more observers.

[0076]FIG. 12 is an optical schematic illustration of an alternativeimplementation of dot sequential color display system 100 in which aholographic optical element 130 (e.g., grating) that disperses orseparates color components of the light and directs them to a microlensarray 118 is substituted for ACS system 116.

[0077] Holographic optical element 130 cooperates with color filter 93to function as a uniform telecentric color separator, which asillustrated in FIG. 10 means that the pixel apertures of imaging device98 are located at the front focus, resulting in the chief rays beingparallel to the optical axis in the image space (i.e., normal to theplane of imaging device 98 as illustrated). In one implementation,holographic optical element 130 and holographic optical element 122 areidentical and microlens array 118 is of the lenticular type.

[0078]FIG. 13 is an exploded functional illustration of holographicoptical element 130. Holographic optical element 130 includes threeholographic lens grating layers, each for diffracting a differentwavelength range. In the illustrated implementation, Δ n and thicknessare controlled to give each layer a bandwidth of 60-75 nm, for example.

[0079]FIG. 14 is an optical schematic illustration of a resolutionenhancing dot sequential color display system 200 according to thepresent invention. Dot sequential color display system 200 illustrates aconventional three-panel color projector configuration that furtherincludes a “dot-shifter,” such as a dynamically tilted plate 202, whichis tilted at a fast field rate to form a four-frame sequence. Forexample, dot sequential color display system 200 includes a pair ofcolor separating dichroic mirrors 204 and 206 that reflect respectiveblue and green light and transmit other light. Fold mirrors 208, 210,and 212 redirect the color separated light components toward monochromedisplay devices (e.g., LCDs) 214, 216, and 218. An X-cube prismcombination 220 combines the color component images, which pass throughdynamically tilted plate or wobbler 202 to a projection lens assembly222.

[0080]FIG. 15 illustrates the resolution enhancement provided by displaysystem 200. Display image pixel 230 illustrates the overlapping colorcomponent sub-pixels provided by a conventional three-panel colorprojector configuration (The color component sub-pixels are shown withslight offset for purposes of illustration). Display image pixel 232illustrates one implementation of four non-overlapping color componentsub-pixels 234 provided by dynamically tilted plate 202 of displaysystem 200.

[0081]FIG. 16 shows an expanded non-overlapping pixel 236 with anexemplary sub-pixel displacement sequence 238 (sometimes referred to asa “bowtie” sequence) that includes alternating diagonal and verticalsub-pixel displacements. Sequence 238 is preferable over circumferentialsequence (e.g., sub-pixels A, D, B, and C) because the alternating rowsof sequence 238 are compatible with interlaced display formats, such asstandard television. FIG. 17 further illustrates the resolutionenhancement provided by display system 200. Pixels 240 illustrate aconventional original four full-color pixels without displacement.Pixels 242 illustrate 16 apparent full-color pixels that are provided bysub-pixel displacement. The non-overlapping arrangement of colorcomponent sub-pixels 242 provides an enhanced image resolution. (The 16pixels are shown with slight offsets for purposes of illustration).

[0082] FIGS. 18-22 are diagrammatic illustrations of dynamicpost-display pixel element alignment systems or “wobblers.” FIG. 18shows two prism arrays 250 and 252 with two piezoelectric actuatorstacks 254 and 256 and a voltage waveform that is applied topiezoelectric actuator stacks 254 and 256. The voltage waveformcooperates with piezoelectric actuator stacks 254 and 256 to separateprism arrays 250 and 252 by different distances so as to shift colorcomponent dots into alignment to get the desired R, G, B superposition.

[0083]FIG. 19 illustrates a dynamic post-display pixel element alignmentsystem or “wobbler” that includes a flat doubly birefringent crystal 260(e.g., calcite) with a crystal polarization direction 262. Crystal 260and its polarization direction are rotated about a central axis 264.Crystal 260 is illustrated in combined front and side views atsuccessive times t1, t2, and t3.

[0084] In the illustrated implementation, light has a horizontalpolarization so that it passes through crystal 260 without displacementwhenever crystal polarization direction 262 is horizontal, as at timet1. Whenever crystal polarization direction 262 is vertically upward, asat time t2, light passes through crystal 260 with an upwarddisplacement. Whenever crystal polarization direction 262 is verticallydownward, as at time t3, light passes through crystal 260 with adownward displacement. As crystal 260 rotates, the light will insequence be displaced in one direction, pass straight through, bedisplaced in the opposite direction, pass straight through, etc. As aresult, image resolution can be tripled to form a pixel pattern 264.

[0085]FIG. 20 is a front view of a dynamic post-display pixel elementalignment system or “wobbler” that includes a wheel 270 with four flatrefractive segments A, B, C, and D with different angled orientations todisplace the light in four different directions. Segmented wheel 270 isrotated about a central axis 272 so that the light propagating along apath 274 passes through the angled segments successively. FIG. 21 is aside view of wheel 270 showing segment A displacing light in a downwarddirection. It will be appreciated by the orientation of segment B thatit will displace light in an upward direction when segment B ispositioned across path 274. FIG. 22 is a top view of wheel 270 showingsegment C displacing light in a rightward direction. It will beappreciated by the orientation of segment D that it will displace lightin a leftward direction when segment D is positioned across path 274.

[0086] Although illustrated in a four-segment implementation, it will beappreciated that wheel 270 could alternatively be implemented as threesegments, rather than four. In one three segment implementation, forexample, a first and a third segment could have the orientations ofsegments C and D, and an intervening second segment could be orientedwith no angular tilt (i.e., perpendicular to path 274).

[0087]FIG. 23 is a diagrammatic illustration of an alternative colormosaic arrangement 300 of sub-pixels 302 in a display device (e.g.,LCD). Implementations described above refer to arrangements in which thesub-pixels of each color component are arranged in distinct verticalcolumns. Color mosaic arrangement 300 positions sub-pixels 302 in adenser, closer-packed arrangement that provides improved imagecharacteristics because the human eye sees the staggered, offset pixelarrangement as having a higher spatial resolution, particularly in thehorizontal direction. In addition, television signals may also besampled with offsets by using alternating clock edges (e.g., chrominancesignals).

[0088] As with implementations described above, a reflector 12 collectsgenerally white light from a lamp 14 and directs the light through anangular color separation system 304 that provides regular angle colorseparation in which red, green and blue color components are separatedacross one axis (e.g., horizontal). In this implementation, the light isshown passing through a microlens 306 having an elongated, close-packed(e.g., hexagonal) configuration in which each microlens 306 is alignedwith a full-color triplet of sub-pixels 302. As a result, themicrolenses 306 at the display device (e.g., LCD) turn the horizontalangular separation of color into LCD pixel separation. Linear (e.g.,horizontal) dot sequential modulation 308 displaces light horizontallyduring three successive times in each frame to provide a completedisplay image in accordance with the present invention.

[0089]FIG. 24 is a diagrammatic illustration of an alternative colormosaic arrangement 310 of sub-pixels 312 in a display device (e.g.,LCD). Color mosaic arrangement 310 positions sub-pixels 312 in anotherdense, closer-packed arrangement that differs from arrangement 300 inthat the former includes one sub-pixel 312′ (e.g., a center pixel,illustrated as receiving green light) that is offset from alignment withthe other two sub-pixels 312.

[0090] As with implementations described above, a reflector 12 collectsgenerally white light from a lamp 14 and directs the light through anangular color separation system 314 that provides angle color separationin which red and blue color components are separated from each otheracross one axis (e.g., horizontal) and from green across two axes (e.g.,horizontal and vertical). In this implementation, the light is shownpassing through a microlens 316 having a regular, close-packed (e.g.,hexagonal) configuration in which each microlens 316 is aligned with afull-color triplet of sub-pixels 312. As a result, the microlenses 316at the display device (e.g., LCD) turn the angular separation of colorinto LCD pixel separation. Triangular or circular dot sequentialmodulation 318 displaces light during three successive times during eachframe to provide a complete display image in accordance with the presentinvention.

[0091] In view of the many possible embodiments to which the principlesof our invention may be applied, it should be recognized that thedetailed embodiments are illustrative only and should not be taken aslimiting the scope of our invention. Rather, the invention includes allsuch embodiments as may come within the scope and spirit of thefollowing claims and equivalents thereto.

1. A color display system, comprising: an illumination system thatprovides fixed, color-separated illumination of color-componentsub-pixels in a pixellated electronic display panel; and a post-displaypanel dynamic displacement element that displaces alignment of thecolor-component sub-pixels generated by the display panel.
 2. The systemof claim 1 further comprising an angular color separation system withplural angularly inclined dichroic mirrors for providing the colorseparation of incident multi-color illumination light.
 3. The system ofclaim 1 further comprising a microlens array positioned adjacent thepixellated electronic display.
 4. The system of claim 3 furthercomprising a grating positioned between the microlens array and thepixellated electronic display.
 5. The system of claim 4 in which thegrating includes a holographic optical element.
 6. The system of claim 1further comprising a grating for providing the color separation ofincident multi-color illumination light.
 7. The system of claim 6 inwhich the grating includes a holographic optical element.
 8. The systemof claim 1 in which the dynamic displacement element includes a rotatingelement that successively directs the color-component sub-pixelsgenerated by the display panel along different optical paths.
 9. Thesystem of claim 1 in which the rotating element includes a birefringentelement with a selected polarization direction.
 10. The system of claim1 in which the rotating element includes a plural refractive segmentshaving different inclination orientations.
 11. The system of claim 1 inwhich the dynamic displacement element includes a pair of face-to-facerefractive elements with a separation between them that is modified tosuccessively direct the color-component sub-pixels generated by thedisplay panel along different optical paths.
 12. The system of claim 11in which each of the refractive elements includes a prism array.
 13. Thesystem of claim 1 further comprising a color separating element forproviding the color separation of incident multi-color illuminationlight and a prism array positioned after the color separating element.14. The system of claim 13 in which the color separating elementincludes an angular color separation system with plural angularlyinclined dichroic mirrors.
 15. The system of claim 1 in which thedisplay panel includes color-component sub-pixels that are arranged invertical columns for each color component.
 16. The system of claim 1further comprising a microlens array positioned adjacent the displaypanel, wherein the each microlens is aligned with and delivers light toa triplet of color-component sub-pixels that are arranged in ahorizontal row.
 17. The system of claim 16 in which the display panelincludes color-component sub-pixels that are arranged in verticalcolumns for each color component and successive sub-pixels in eachcolumn are positioned in alternate successive rows.
 18. The system ofclaim 1 in which the display panel includes color-component sub-pixelsthat are arranged in vertical columns for each color component and thesystem further comprises a microlens array positioned adjacent thedisplay panel, wherein the each microlens is aligned with and deliverslight to a triplet of color-component sub-pixels that are arranged thatare positioned among two adjacent horizontal rows.
 19. A colorelectronic display projector, comprising: an illumination system thatprovides fixed, color-separated illumination of color-componentsub-pixels in a pixellated electronic display panel; and a post-displaypanel dynamic displacement element that displaces alignment of thecolor-component sub-pixels generated by the display panel.
 20. Theprojector of claim 19 further comprising an angular color separationsystem with plural angularly inclined dichroic mirrors for providing thecolor separation of incident multi-color illumination light.
 21. Theprojector of claim 19 further comprising a microlens array positionedadjacent the pixellated electronic display.
 22. The projector of claim21 further comprising a grating positioned between the microlens arrayand the pixellated electronic display.
 23. The projector of claim 22 inwhich the grating includes a holographic optical element.
 24. Theprojector of claim 19 further comprising a grating for providing thecolor separation of incident multi-color illumination light.
 25. Theprojector of claim 24 in which the grating includes a holographicoptical element.
 26. The projector of claim 19 in which the dynamicdisplacement element includes a rotating element that successivelydirects the color-component sub-pixels generated by the display panelalong different optical paths.
 27. The projector of claim 19 in whichthe rotating element includes a birefringent element with a selectedpolarization direction.
 28. The projector of claim 19 in which therotating element includes a plural refractive segments having differentinclination orientations.
 29. The projector of claim 19 in which thedynamic displacement element includes a pair of face-to-face refractiveelements with a separation between them that is modified to successivelydirect the color-component sub-pixels generated by the display panelalong different optical paths.
 30. The projector of claim 29 in whicheach of the refractive elements includes a prism array.
 31. Theprojector of claim 19 further comprising a color separating element forproviding the color separation of incident multi-color illuminationlight and a prism array positioned after the color separating element.32. The projector of claim 31 in which the color separating elementincludes an angular color separation system with plural angularlyinclined dichroic mirrors.
 33. A color display method, comprising:illuminating color-component sub-pixels in a pixellated electronicdisplay panel with color-separated, fixed color components; anddynamically aligning the color-component sub-pixels after the displayelement.
 34. The method of claim 33 further comprising an angularlycolor separating incident multi-color illumination light to provide thecolor-separated color components.
 35. The method of claim 33 in whichdynamically aligning the color-component sub-pixels includessuccessively directing the color-component sub-pixels generated by thedisplay panel along different optical paths.
 36. The method of claim 35further comprising successively directing the color-component sub-pixelsthrough different segments of a rotating light displacement element. 37.The method of claim 33 in which the display panel includescolor-component sub-pixels that are arranged in vertical columns foreach color component and dynamically aligning the color-componentsub-pixels after the display element includes displacing selected colorcomponents laterally.
 38. The method of claim 33 in which thecolor-component sub-pixels of a pixel are arranged on the display panelin adjacent rows and dynamically aligning the color-component sub-pixelsafter the display element includes displacing selected color componentsin transverse directions.
 39. In a color display system with pluralpixellated electronic display panels that each receive illumination of adifferent color component of light and a combiner that combines colorcomponent light images formed by the display panels, the improvementcomprising: a post-combiner dynamic displacement element that displacesalignment of the color-component sub-pixels generated by the displaypanel to form a resolution-enhanced display image.